A variety of electrical sensors for sensing the relative position of an object are known in the art. Such sensors typically operate by varying the magnitude or frequency of an electric voltage, current or magnetic field as a function of the position of the object being sensed. While such electrical sensors work well for many applications, there are environments where they cannot be used. For example, because electrical sensors are susceptible to electrostatic and electromagnetic interference and coupling, they are not suited for use in the presence of strong electrostatic and electromagnetic fields. Also, electrical sensors may cause a spark, thereby posing a risk of explosion when placed in areas containing highly volatile vapors. Optical sensors, on the other hand, offer the benefits of being lightweight, immune to electrostatic and electromagnetic fields and are usable in the presence of explosive vapors. Therefore, optical sensors are being used more and more where electrical sensors had previously been used.
A typical optical sensor includes a rotatable encoder plate that is mechanically coupled to the object whose position is to be sensed such that the angular position of the encoder plate is controlled by the position of the object. A light source directs an incoming beam of light onto the encoder plate and a read head detects an outgoing beam of light having an intensity that is a function of the position of the encoder plate, thereby providing an indication of the position of the object.
One significant problem with prior art optical position sensor designs is the presence of an air gap that exists between the read head and the encoder plate. This air gap is the source of several problems. First, any loading of the mechanical linkage that couples the encoder plate to the object whose position is to be sensed can cause an angular misalignment between the encoder plate and the read head. For some sensors, an angular misalignment of only a few minutes of an arc can increase the optical path loss by several decibels. This variation in the optical path loss affects the intensity of the outgoing light beam and is indistinguishable from a change in intensity due to movement of the object. Similarly, mechanical loading of the linkage can also cause a change in the width of the air gap. An increase in the air gap by only a few thousandths of an inch also can increase the optical path loss by several decibels. Furthermore, the air gap provides an opportunity for contaminants to enter the optical path. If contaminants are present, they can cause total optical path loss, thereby rendering the sensor inoperative. Finally, the air gap between the read head and the encoder plate present refractive index discontinuities, which cause significant optical path losses unless suitable anti-reflective coatings are used. Similar problems exist with optical sensor designs that include a linear, as opposed to a rotary encoder plate. Further, the problems exist in both reflective and transmissive optical position sensors.
The present invention is directed to avoiding the problems associated with prior art optical sensor designs having an air gap between the read head and the encoder plate. Specifically the present invention is directed to providing an optical sensor that minimizes, if not entirely eliminates, angular misalignment between a stationary read head and a moving encoder plate. The present invention is also directed to providing an optical sensor that eliminates the possibility of contamination between optical surfaces and losses due to refractive index mismatch of the read head, encoder plate and air gap.